Joy is more than its own reward.
Happy people are healthier.
Happy bodies fight infections better.
Happy brains think better.
We cannot always be happy. In fact life often makes us sad. Sadness can make life worse. When we are sad our faces droop. When we are sad our bodies droop. We have less interest in life and living. We might even have trouble getting out of bed. When you see a sad person do you want to say hi, or avoid them? When others ignore you does that make you feel happier or sadder? Thus, being sad can bring its own spiral of increasing sadness.